Q. Which one of the following types of glass can cut off ultraviolet rays? (UPSC Prelims 2007)
Answer: Crookes glass
Notes: Crookes glass, developed by Sir William Crookes, is specially formulated with rare earth oxides like cerium to strongly absorb ultraviolet (UV) rays, particularly UVC and UVB, while transmitting visible light. This makes it ideal for protective eyewear and laboratory applications against UV radiation. In contrast, soda glass (soda-lime) transmits most UV; Pyrex (borosilicate) blocks some but not effectively; Jena glass (similar borosilicate) offers limited UV protection. Historical records confirm Crookes glass's UV-blocking properties since the late 19th century, used in sunglasses and scientific instruments.